[seL4] Re: Runtime Measurement Hiccup

27 Oct 2021

      Gernot and all,

No, you have it all right. It's only that I'm frustrated because I cannot
see the implementation of what everyone so offhandedly calls easy.
I can see how to grant R/O access to a part of the user-level address
space, but I don't see how to grant R/O access to a useful address space.
I've asked about it here before, and I spent quite a long time bringing
kernel modules to the virtualized-linux space in an effort to realize this
end.
In my inexperience, I took it personally when I was told what I spent so
long creating was worthless for this effort, but in a lasting way I realize
knowledge has value in its own right.
Hugo is obviously well-experienced and knowledgeable, and I respect his
opinion highly.

I'm sure I will come back with a more appropriately worded question after
going back to the source, or maybe I will surprise you with some amusing
solution.

Sincerely,
Michael Neises

On Tue, Oct 26, 2021 at 12:07 PM Gernot Heiser  wrote:
...
Folks,
I’m not sure what triggered that reaction of Michael’s quoted by Hugo
below, but it must have been something off-list. Certainly the discussion I
saw on the list was perfectly polite and constructive, let’s keep it that
way please.
In terms of the technical issues, I can only agree with Hugo: I fail to
see how the guest measuring itself can give you any integrity guarantee. If
you assume the guest to be compromised (and why else would you want to
measure it) then you have to also assume it to be arbitrarily malicious,
and thus it could just fake the measurement and return a known “good value”
that has nothing to do with the correct measurement.
To ensure integrity, the measurement has to be done outside the guest. And
doing that should not be hard: Have a separate measurement component that
has R/O access to all of the guest’s address space, and it can perform the
measurement in a tamper-proof fashion.
Am I missing something?
Gernot
...
On 26 Oct 2021, at 05:29, Hugo V.C.  wrote:
Hi Michael!
Adding the full list to the thread again (we missed them at some
point...).
"I appreciate the complete lack of partial credit, and I consider you
find
my work to be a waste."
I don't think anyone thinks your work is a waste. Personally, I just gave
my opinion about the architecture, but of course the final decision is
yours, as it is your baby :)
Maybe someone on the list can answer you with a more specific example you
require (even if they already gave some hints...).
Cheers,
El lun, 25 oct 2021 a las 19:11, Michael Neises (<
neisesmichael@gmail.com>)
escribió:
...
Hugo (and Everyone),
Thank you for the reminders.
I believe it's trivial to provide or restrict caps to any IO device. So,
yes, I believe with the board's reference manual it should
theoretically be
quite easy to restrict them all.
I appreciate the complete lack of partial credit, and I consider you
find
my work to be a waste. So with that in mind I'll ask once more:
Has anyone on the planet ever performed such an independent measurement
of
a virtual machine, or is seL4 really as unusable as indicated?
I don't want to spend any amount of time barking up a tree that doesn't
exist.
Cheers,
Michael Neises
On Mon, Oct 25, 2021, 01:02 Hugo V.C.  wrote:
...
Hi Michael,
as I commented, it depends on the runtime environment. The scenario I
described, even if challenging, it is just one of many you could face.
Let
me explain myself.
Really, it is irrelevant if the full Linux (or whatever OS) VM is
"inmutable". At some point you need to load code into memory and run
it.
Then, only formal verified code (like seL4) is reasonably secure. Being
pedant, anything else simply it is not.
Why?
The reason is you will never be sure what interactions the VM OS (in
your
example Linux) will have with the outside World. Do you have NTP
client...?
HTTP clients...(wget)? DNS clients...? Are you absolutely sure you know
every line of this VM OS (Linux) and can guarantee there will be no
out of
control interaction with the outside Word?
Let's go a step ahead in the offensive mindset. Even in the case you
are
building a siloed "air gap" machine (no networking), do you have full
awareness of all the I/O mechanisms of the device so you can guarantee
there will be no interaction with the outside World...?
For that reasons code is formally verified. That is the only way to be
sure things are reasonably secure.
If we accept the last statement as true, any integrity check done from
inside of unverified code, is, by definition, not trustable. But of
course
you can do it.
On the other side, what I don't get is, if you consider (for whatever
reason) your guest OS is inmutable... then why you want to check
integrity
from inside...?
In embedded World, integrity checks always need something
(theoretically)
really inmutable (i.e. CPU fuses). You need to check/anchor from the
most
trustable source you have. That's why in embedded devices there are
those
"funny" boot sequences with chain of trust where different parts of the
system (from most simple to most complex) are used to verify the next
step
in the boot chain.
Having said that, of course you can do integrity checks from inside the
VM itself, but IMHO will be a waste of trusted computing power of seL4
platform.
Please excuse me in advance if I misunderstood your message.
A very interesting topic.
Cheers,
El lun., 25 oct. 2021 2:34, Michael Neises 
escribió:
...
Hugo and Everyone,
Thanks for the response. This is something I've worried about as well.
I've been under the impression that once I compile a seL4 image, that
image should be static no matter how many times I boot it. That is,
I've
looked around for persistent storage to use, and my solution has so
far
been to recompile the entire seL4 image in order to insert new data.
So
even when I "touch" files in the Linux virtual machine, they are
completely
forgotten when I reboot the system. For a time I thought of this as an
impediment, but I soon came to realize it as a benefit. So I suppose I
should clarify that when I said "Linux kernel" in that quote, I really
meant this particular Linux image which is prepared at compile-time
and
virtualized by seL4 at runtime.
For the last several months, I've been operating under the assumption
that there is no way for me, even as a developer, to "manipulate the
seL4
image I used to boot myself." Namely, I've been trying to jump
through all
these virtual network hoops because I couldn't figure out a way to
make
persistent changes to the image. So, as I said, I had taken it for
granted
that a seL4 image was immutable in this way, but I recognize your
point
that maybe it is not. My argument has been that the seL4 image is
loaded
onto an SD card, and I can forbid access to that SD card, which means
the
image should be guaranteed to be untouchable except maybe by the seL4
kernel itself.
I believe seL4's proofs uphold my argument regarding "capabilities" to
the SD card, but I admit a slim understanding of seL4's "caps." I
will be
happy as always to be edified.
Cheers and Good Evening to you,
Michael Neises
On Sun, Oct 24, 2021 at 4:32 PM Hugo V.C. 
wrote:
...
Hi Michael,
"Please correct me if I am wrong, but I think if the very first thing
the Linux kernel does is measure itself, before it is even connected
to a network, then there is simply no attack surface"
My 5 cents: it is not so simple... it depends on the specfic run time
environment.
Anyway, just as an example, some years ago I was challenged with a
similar scenario: an appliance running Linux firmware with an
embedded
integrity mechanism in the kernel code that checked its own
integrity and
also the integrity of all loaded kernel modules (that were doing
integrity
checks of the file system). Once initial modules were loaded no more
were
allowed to be loaded.
Anyway, the running kernel was very outdated, so I was able to find a
vulnerability that allowed me to inject my own data/code in the
kernel
space. The problem was persistence: most of the file system was read
only... with the exception of some config files in the compact flash
storage... a second bug in the parsing of the config files (that
allowed
user space command execution to trigger the kernel vuln) gave me the
persistence I wanted for my kernel level vulnerability in that
"inmutable"
system. Game over.
So, it really depends on your environment. As long you have I/O data
operation were an attacker can interact to some persistent storage,
then
there's room for persistent intrusion no matter the runtime checks
you do
on the kernel or the file system. There have been plenty of even more
elaborated attacks/tricks on heavily siloed and isolated and
"inmutable"
systems that have been carried out in the computing history. In
fact, those
are the interesting ones... :-)
Hopes this helps.
El dom., 24 oct. 2021 19:46, Michael Neises 
...
...
...
...
escribió:
> Hello seL4 developers,
>
> Thank you for the replies.
>
> For the sake of clarity, the system works like this:
> At compile time, some expected digest values are made available only
> to a
> distinct CAmkES component. At the time of first-Linux-boot, a kernel
> module
> takes several measurements of the other kernel modules present
> (including
> itself). It reports these digests outwards to CAmkES, where they are
> compared against the expected values. It is the "pre-compile-time
> provisioning of these expected digests" in which I am interested. At
> this
> time, I can simulate the system and compute these digests, but the
> only way
> I have to extract them is to copy them by hand off the screen. To be
> totally explicit, I want to extract these values in order to
re-compile
> them into a system that knows its expected digest values. I want to
> have an
> initial simulation where I extract these digests, so that in the
> subsequent compilation and simulations, the system is aware what
values
> these digests are required to take.
>
> Please correct me if I am wrong, but I think if the very first thing
> the
> Linux kernel does is measure itself, before it is even connected to
a
> network, then there is simply no attack surface. Of course I'm very
> happy
> to be wrong, but I don't see who the attacker is in this situation.
> Certainly, there remains an open question of how to extend these
> measurements meaningfully into the space where there is a viable
attack
> surface (after enabling a network adapter), but I consider that
> question to
> be beside the point for now (some future work). If there is some way
> for me
> to inspect the run-time data of the Linux system without relying
> somewhat
> on a tool inside the Linux instance, I would very much like to know
> about
> it. My strategy follows the same path as the vm-introspect example
app
> (which I'm under the impression was created for this explicit
purpose),
> which itself trusts implicitly the Linux instance. Again, to be
> entirely
> explicit, there does not appear to be any information anywhere on a
> way to
> meaningfully inspect a virtualized Linux system without trusting it
> even
> the slightest bit. I would be elated to be corrected; if someone can
> show
> me how to scrape (from the seL4 side exclusively) all the bits from
a
> virtual Linux system and reassemble those bits into
> semantically-valuable
> information, I'm sure you will not hear from me for several months
> while I
> rebuild everything I have. At this time that does not appear to be
> possible.
>
> Cheers,
> Michael Neises
>
> On Tue, Oct 19, 2021 at 3:46 PM Michael Neises <
> neisesmichael@gmail.com>
> wrote:
>
>> Hello seL4 developers,
>>
>> I want to be able to retrieve data from seL4's virtual Linux
> machine, in
>> order to store it in a persistent way. Namely, I want to be able to
>> simulate a seL4 kernel, boot its Linux virtual machine, compute
some
> hash
>> digests, and then export those hash digests. These digests are
> valuable
>> because they represent the "clean room" runtime-state of the linux
> machine.
>> Currently I can export these digests by way of hand-eye
> coordination, but I
>> consider this unusable as a piece of software.
>>
>> To date I've taken two main approaches: CAmkES FileServer or
virtual
>> networking. I'm under the impression that the FileServer changes
are
> not
>> persistent through reboot, and even if they were, to change the
boot
> image
>> after compile-time would seem to fly in the face of seL4's
> principles.
>> Virtual networking seems to promise I can host my digests on a
> webpage that
>> is visible to my "root host" machine; that is, the simulated seL4's
> linux
>> instance hosts a site available on my 192.168.x.x network. I know
> there is
>> a seL4webserver app as part of the seL4 repositories which claims
to
> do
>> this, but unfortunately its prose is unhelpful and it doesn't seem
> to work
>> even when it compiles and simulates.
>>
>> I've taken two distinct strategies to investigate the virtual
network
>> approach. First, I tried to get it to work on my normal stack:
> Windows 10
>> using WSL2 using a Docker container to simulate the seL4 image. The
> problem
>> with this approach is that it appears I'm required to blindly
thread
> 3 or 4
>> needles all at once, without getting feedback more descriptive than
> "you
>> didn't do it." In other words, there does not appear to be a
partial
>> success available, and without ICMP ping, I honestly have no idea
> how to
>> debug these "virtual" networks.
>>
>> Next, I tried simplifying my stack by installing the dependencies
> natively
>> on a Debian 10 machine, which should bypass several layers of the
> virtual
>> network I was suggesting in my first strategy. Unfortunately, I met
> with
>> the same "AttributeError: module 'yaml' has no attribute
> 'FullLoader'"
>> error that inspired me to begin using Docker several years ago. Of
> course I
>> should note that "pip/pip2/pip3 install pyyaml" all report that
> pyyaml is
>> already installed, so I would be in debt to anyone who has an idea
> about
>> that error.
>>
>> To conclude, I find virtual networks opaque, and I would be
grateful
> for
>> any guidance. If you have a different idea how I might achieve my
> goal, I
>> would be similarly effusive in my thanks.
>>
>> Cheers,
>> Michael Neises
>>
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